Ultrafiltration processes for the recovery of polymeric latices from whitewater

ABSTRACT

Polymer is recovered by ultrafiltration from a whitewater waste stream generated during the production of a polymer latex. The whitewater stream is circulated through an ultrafiltration system in laminar flow, under conditions of shear insufficient to destabilize the whitewater emulsion, and the recovered polymer is in the form of an emulsion which may be blended at significant levels into the original polymer latex without degrading its performance properties.

This is a divisional of application Ser. No. 695,863, filed May 6, 1991now U.S. Pat. No. 5,171,767.

BACKGROUND OF THE INVENTION

Polymer latices, also termed polymer emulsions, are widely used inindustrial applications, including binders for paints, printing inks,non-woven fabrics and the like, paper coatings and the like. Theselatices may be prepared in continuous or batch processes by polymerizingmonomers, usually ethylenically unsaturated compounds, in the presenceof water, surfactants and other adjuvants that affect the manufacturingprocess or the properties of the latices.

Economics may dictate that the same kettles, piping and other equipmentbe used to produce different latices, so the equipment must be cleanedbetween batches. Even where a single latex is produced on a continuousbasis, the equipment must still be cleaned periodically.

Cleaning usually comprises washing the equipment with water; thiscreates large volumes of dilute aqueous latex known as whitewater.Whitewater thus created normally has a solids concentration of about 5%by weight or less, although it may be higher. This solids concentrationrepresenting emulsion-sized particles of the original polymer product.In addition to these submicroscopic polymeric particles of the latex,whitewater may also contain alcohols or other organic liquids,surfactants and the like. As produced, the solids concentration of thewhitewater emulsion is far below the typical 40% or greater found in theoriginal polymer latex, but it represents enough suspended organicmatter to cause a serious waste-disposal problem.

Typical whitewaters may contain emulsion-sized particles of polymerssuch as styrenics, acrylics such as polymers of esters of acrylic ormethacrylic acids, acrylonitrile, vinyl polymers such as poly(vinylchloride), and complex copolymers of two or more such materials, withcrosslinkers, graftlinkers and the like, such as butadiene,divinylbenzene, ethylene glycol dimethacrylate, allyl methacrylate andthe like.

In typical manufacturing operations, the whitewater generated by batchesof different polymer types are combined, and the entire mixture istreated as a single waste stream. To reduce the volume of waste, thewhitewater is frequently concentrated before disposal, typically bychemical coagulation, coarse filtration, and in some casesultrafiltration. The concentrated or coagulated waste, which is amixture of whatever polymers the equipment happened to be making, pluscleaning agents and miscellaneous contaminants, is then typically buriedin land-fill, or used as filler in asphalt or as a dust-control agent onroadways.

Semipermeable membrane filtration, and in particular ultrafiltration,has been employed to concentrate polymer emulsions or latices. In theultrafiltration process a latex is pumped into the inlet end of a hollowmembrane fiber, or cartridge comprising several of these fiber inparallel; the walls of these tubes are "semipermeable", that is, theyallow materials of low molecular weight to pass through, but areimpermeable to higher-molecular-weight materials such as polymer. Thepumped latex flows through the hollow lumen of the membrane fiberparallel to its walls; this flow is known as "cross flow". As the latextransits the lumen of the membrane fiber, water, salts, surfactants andother low-molecular-weight materials pass from the latex through thewalls of the membrane. The flow rate through the membrane wall per unitof membrane surface area is the membrane "flux", and the liquid whichhas passed through the membrane wall is called the "permeate". Thepolymer and other high-molecular-weight materials which do not passthrough the membrane wall appear in the "retentate", which emerges alongwith some of the water from the exit end of the membrane fiber orcartridge under the pumping pressure, and is recycled through the fiberor cartridge until the desired concentration is reached. Transmembranepressures, that is, pressures across the membrane wall, are typicallyfrom about 70 to about 1400 kiloPascals (kPa), more typically from about140 to about 700 kPa. Hydrodynamic pressures, that is, pressures acrossthe length of the membrane fiber or cartridge, depend upon the viscosityof the latex at the operating temperature, and are typically in the samerange as for the transmembrane pressures. Temperatures for theultrafiltration process are typically within the range of about 5° C. toabout 70° C., and more typically about 10° C. to about 40° C.

The above description of ultrafiltration is based upon theultrafiltration membrane being configured as a hollow fiber.Ultrafiltration membranes may also be configured as larger tubes or assheets, which may be used singularly or in pairs with the activemembranes facing one another and the liquid to be treated being passedbetween them; such sheets may be used flat or wound into spiral tubes.Other configurations are known to those skilled in the art.

The latex is sheared as it is pumped through the ultrafiltration system.Sources of shear include the pump or other device used to propel thewhitewater through the system, and the ultrafiltration cartridge itself;shear occurs as the whitewater is forced under pressure into therelatively small inlet port or ports into the cartridge, and as thewalls of the membrane resist the flow of the whitewater. This mechanicalshearing contributes to destabilizing the latex and forming a coagulum,or aggregate of polymeric latex particles, which fouls the membranesurface and pores, reducing flux rate through the membrane. Theultrafiltration process also removes some of the water from the aqueousphase and removes surfactant from the polymer latex, which also helpsdestabilize the latex. Such destabilized latices do not retain theiroriginal performance properties, and must be regarded as low gradeproduct or waste.

As a result, previous attempts at concentrating whitewater byultrafiltration produced mixed success, because many latices proved tobe unsuited for the process The flux, which was initially satisfactory,deteriorated rapidly because of the fouling described above. Themembranes required frequent cleaning, for instance by washing them withsurfactants or solvents, as described in U.S. Pat. No. 3,956,114, toremove the fouling and at least partially restore the flux rate. Thisfrequent cleaning not only removed the system from service, reducing theoverall throughput of the system, but also was only partially effective,so the overall life of the membrane filter was often unsatisfactorilyshort.

In U.S. Pat. No. 4,160,726, the problem of coagulum formation, as itrelates to fouling, was addressed by adding surfactant to the whitewaterlatex prior to or during the concentration process, in an attempt tostabilize the latex. While partially successful, this approach did notnecessarily work for all whitewater latices, and did not address thechange in properties of the retained latex.

An object of the present invention is to provide a process by which thepolymer latices recovered from whitewater may be recycled intohigh-value product instead of being treated as low-value waste andby-product. Another object of the present invention is to provide anapparatus to recover such high-value product, and yet another object isto provide the high-value, polymeric product so recovered. Other objectsof the invention will be apparent from the specification and claimswhich follow.

SUMMARY OF THE INVENTION

We have discovered a process for recovering a polymer latex product froma whitewater emulsion which comprises the steps of

(a) contacting the whitewater emulsion with an ultrafiltration membranehaving an active membrane side and a porous support side so that theemulsion flows in laminar flow across the active membrane side under apressure higher than the pressure on the porous support side, to removewater from the emulsion,

(b) recirculating the emulsion such that it flows in laminar flow acrossthe active membrane side repeatedly until the emulsion has beenconcentrated to a solids content of 20 weight percent or greater, and

(c) returning the concentrated emulsion to the polymer latex product,wherein the whitewater emulsion is subjected to shear insufficient todestabilize the whitewater emulsion, and wherein the whitewater emulsionis a byproduct formed by diluting the polymer latex product with anaqueous liquid. The concentrate from this process is of good quality andmay be blended with the product streams without affecting the propertiesof the products.

We have further discovered an apparatus for recovering a polymer latexproduct from a whitewater emulsion which is a byproduct formed bydiluting the polymer latex product with an aqueous liquid, illustratedin FIGS. 1 and 2, which apparatus comprises

(a) one or more ultrafiltration cartridges having disposed therein oneor more ultrafiltration membranes having an active membrane side, thecartridges further having a whitewater inlet and a whitewater outletsuch that a liquid flowing between the inlet and outlet will flow, inlaminar flow, in contact with the active membrane side of the membrane,and the cartridges further having a permeate outlet, the membraneseparating the permeate outlet from liquid flowing between thewhitewater outlet and the whitewater inlet,

(b) a whitewater inlet line for the flow of liquid, the line having adischarge end and an input end, the discharge end being connected to thewhitewater inlet of the ultrafiltration cartridges,

(c) a pumping means for a liquid, the pumping means having an inlet andan outlet, the outlet of the pumping means being connected to the inputend of the whitewater inlet line and the inlet of the pumping meansbeing connected to a source of whitewater,

(d) a return line for the flow of whitewater connected from thewhitewater outlet of the ultrafiltration cartridges to the source ofwhitewater, the inlet of the pumping means, or the connection betweenthem, the path for liquid flow from the inlet of the pumping meansthrough the pumping means, through the whitewater inlet line, throughthe ultrafiltration cartridges from the whitewater inlet to thewhitewater outlet of the cartridges and thence through the return lineback to the source of whitewater, the inlet of the pumping means, or theconnection between them, forming a recirculation loop.

(e) a first pressure control means disposed within the recirculationloop between the inlet of the pumping means and the whitewater inlet ofthe cartridges for controlling the pressure differential betweenwhitewater inlet and the whitewater outlet of the cartridges to maintaina total shear on the whitewater emulsion in the entire apparatus below alevel which will destabilize the whitewater emulsion.

We have further discovered a polymer latex recovered from whitewateremulsion generated as a byproduct during manufacture of a polymer latexproduct by diluting the product with an aqueous liquid, the polymerlatex being recovered by contacting the whitewater emulsion with anultrafiltration membrane having an active membrane side and a poroussupport side so that the emulsion flows in laminar flow across theactive membrane side under a pressure higher than the pressure on theporous support side, to remove water from the whitewater emulsion,without adding additional surfactant to said polymer, the polymer latexhaving essentially the same physical properties as the polymer latexproduct from which it was generated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simple embodiment of the apparatus suitable for carryingout the process of the present invention, including the ultrafiltrationcartridges (1) with their whitewater inlets (6), whitewater outlets (7)and permeate outlet (8), the whitewater inlet line (2), the pumpingmeans (3), and whitewater source (4). Two cartridges are shown, but theinvention as contemplated includes embodiments wherein only onecartridge is used, or wherein a plurality are used, in parallel asshown, in series, or in a series configuration of parallel cartridges.

FIG. 2 shows a preferred configuration of the apparatus of the presentinvention, wherein a pre-filter (9), preferably a bag filter orequivalent filter, is placed in the whitewater inlet line to removelarge particulate matter that may foul the cartridge membranes, arecirculating line (5) is provided to allow the whitewater torecirculate from the whitewater outlet of the cartridge to the inlet ofthe pump and a smaller, auxiliary source (10) of whitewater is provided.Control valves (11-22) are also indicated which permit the apparatus tobe operated in the manner described below. A bleed line (11) is alsoshown which allows any retentate which is bled off, to reduce pressurein the system, to be returned to the whitewater source tank.

DETAILED DESCRIPTION OF THE INVENTION

We have found a process by which whitewater latices initially containingfrom about 1% or less, and up to about 20%, of polymer solids from asingle polymer latex batch, or a series of batches of a single polymertype, may be segregated and treated by ultrafiltration to concentratethe polymer to solids levels of from about 20% to about 50%, and torecover useable product. A particularly important aspect of the presentinvention involves carefully controlling shear in the ultrafiltrationsystem, particularly when the whitewater emulsion becomes concentratedand relatively viscous.

The whitewater to be treated according to the process of the presentinvention is preferably that generated by cleaning the equipment used tomanufacture a single polymer latex batch, or a series of batches of asingle polymer type. The volume of whitewater is preferably kept assmall as practical, to minimize dilution of the polymer latex, bycontrolling the amount of rinse water, as this reduces the amount ofwater which must be removed by the process of the present invention,which in turn reduces the total time for the ultrafiltration process andthe total shear to which the whitewater is exposed during the process.The solids content of the whitewater is thus preferably about 5 weightpercent or greater, more preferably about 8 weight percent or greater,and still more preferably about 10 weight percent or greater.

Alternatively, the dilution of the polymer latex may be expressed as apercentage of the solids content of the polymer latex. Preferably thedilution of the polymer latex should yield a whitewater emulsion whosesolids content is about 15% or greater, by weight, of the polymer-latexsolids content, and more preferably about 20% or greater, by weight, ofthe polymer-latex solids content.

The critical level of shear in the process of the present invention isthat which causes destabilization of the whitewater emulsion. As thewhitewater is concentrated, it is subjected to shear from the pump andfrom contact with the membrane walls. To reduce the pump shear, the pumppressure is preferably controlled by controlling the speed of the pump.A particularly preferred means for controlling the speed is a frequencyconverter. Alternatively, the pressure may be controlled by a bleedvalve in the line following the pump. Other approaches to controllingpressure from the pump may alternatively be used, and these would bereadily apparent to those skilled in the art.

We believe that, to avoid the deleterious effects of emulsiondestabilization, the shear should be minimized by maintaining the flowacross the ultrafiltration membrane preferably in the laminar domain,that is, the flow across the membrane should be laminar. Another way ofexpressing this desired laminar-flow condition is that the Reynoldsnumber of the whitewater flowing across the membrane is about 3000 orless, preferably about 2100 or less. While a Reynolds number of about2100 or less is generally recognized by those skilled in the art asindicating flow in the laminar domain, the transition between laminarand turbulent flow is not abrupt, and energy input to the whitewateremulsion is still low at Reynolds numbers up to about 3000, which isconsidered for purposes of the present invention to be within thelaminar domain. Maintaining the cross-flow of the whitewater in thelaminar domain reduces the total energy input to the whitewater, andtreats it in a gentler way than prior-art processes, thus keeping thewhitewater emulsion stable.

Further contributing to the gentler processing of the whitewater is theuse of hollow-fiber membranes in the preferred embodiment of the presentinvention. The preferred hollow-fiber membranes have lumens withdiameters of about 3 mm or less, which produces a relatively high ratioof surface area to liquid volume, thereby allowing the removal of morewater from the emulsion in a single pass across the membrane, reducingthe recirculation required to reach a given solids level, and furtherreducing the total energy input to the whitewater emulsion.

The useable product which is recovered from the process of the presentinvention may be blended back into the polymer latex product of which itwas originally a waste by-product, at levels of about 5% or greater,preferably about 10% or greater, more preferably about 20 or greater. Insome cases material with much the same advantageous properties as theoriginal polymer latex is obtained from the present process; suchmaterial essentially represents recovery of the original product fromthe whitewater. Recovery of product which may be sold as-is or blendedat high levels back into the original polymer latex represents asignificant economic benefit to the yield of the manufacturing processfor the polymer latex, reduces the amount of solid material orconcentrated latex which must be disposed of as waste, and reduces theorganic-matter load on water-treatment facilities.

The ultrafiltration system of the present invention is described indetail below.

ULTRAFILTRATION MEMBRANES Cross Flow Filtration

Successful ultrafiltration depends in part upon the high cross-flowvelocity that occurs parallel to the active membrane surface. Thebuildup of polymer or other solids on the membrane wall decreases waterflux through the membrane. Because of the loss of water through themembrane, a concentration gradient is formed close to the membrane wall;this effect is termed concentration polarization. As the soluteconcentration in the bulk fluid rises, the solute concentration at themembrane wall reaches a maximum, forming a gel layer. The thickness ofthe gel layer continues to increase as the bulk solute concentrationincreases. Cross flow, as opposed to dead-ended filtration, helps tosweep the membrane surface clean of solute, and minimizes the effect ofthe gel layer.

With the ultrafiltration of whitewater, the polymer solids areprogressively concentrated, thus increasing the viscosity. This rise inviscosity reduces cross flow velocities, at a constant pressure dropacross the length of the cartridge, and consequently the solute layer atthe membrane wall tends to increase in size and resistance. Thus, theflux of permeate is related to the concentration of polymer solids.

Membranes

The semipermeable membranes useful in ultrafiltration may be made from avariety of materials. Although inorganic membranes such as ceramicmembranes, as well as composite materials in which a ceramic membrane issupported by an organic material, or an organic membrane is supported byan inorganic structure, are within the scope of the invention ascontemplated, preferred membranes are those made from synthetic ornatural polymeric materials. These include membranes in which the poroussupport structure is integral to the membrane layer, and those in whichthe membrane layer is cast or otherwise layered onto the porous supportstructure. Particularly suitable for semipermeable membranes in ourprocess are those synthetic polymeric materials which may be cast, spunor extruded into semipermeable membranes, and which are temperatureresistant and solvent resistant.

Other suitable membrane materials include, but are not limited to,depending upon the process, polyamides, such as nylon and aromaticpolyamides, polyphenylene oxides; olefinic resins, such aspolypropylene, polyethylene and the like; sulfones such as polysulfone,polyethersulfone and the like, cellulosics such as, cellulose acetate,cellulose nitrate, mixed cellulose acetate-nitrate and the like,sulfonated polymers such as sulfonated polysulfone, sulfonatedpolyethersulfone and the like. The selected material for thesemipermeable membrane should preferably be suitable for the normalmembrane-preparation processes, that is, it should be capable of beingcast as a thin layer onto a suitable support material, extruded or spuninto tubes, hollow fibers or other suitable structures, either from amelt or from solution in suitable solvents, or otherwise being formedinto membranes. Copolymers made by copolymerizing two or more monomersare also among such suitable polymeric materials, as for examplecopolymers made by copolymerizing acrylonitrile, methacrylonitrile andother ethylenically unsaturated dienes such as isoprene and butadiene,and various acrylates, such as acrylates and methacrylates and otheracrylic resins such as the esters of acrylic and methacrylic acids, asfor example methyl, ethyl, isopropyl and hexyl acrylates andmethacrylates.

Membranes preferred in this invention are anisotropic membranes, morepreferably anisotropic, hollow-fiber membranes, and still morepreferably anisotropic, hollow-fiber membranes made of polysulfone.Anisotropic membranes have a relatively thick, support structure oflarge, open pores, referred to herein as the "porous support", with athin "skin", or active membrane layer which contains the selectivepores, on one side. In a hollow-fiber membrane the porous supportstructure forms the fiber itself, and the active membrane layer formsthe inner surface that defines the hollow core, or lumen, of the fiber.This inner surface is also referred to herein as the "tube side" of themembrane, as opposed to the porous support side, or outer side. Theporous support structure is typically from about 125 μm to about 550 μmthick and the active membrane layer is about 0.1 μm thick. The preferredfibers of the examples had inside diameters of 1.52 to 1.90 mm (60 to 75mil).

Although the process of the invention is described and illustrated belowin terms of hollow-fiber membranes, one of ordinary skill in the artwill readily understand that the process is equally adaptable to otherconfigurations of ultrafiltration membranes, such as, but not limitedto, single, flat-surface membranes, pairs of flat-surface membranes inwhich the active membrane layers face one another and the process streamis passed between them, spiral-wound membrane cartridges, andlarge-diameter membrane tubes. One of ordinary skill in the art willsimilarly understand the manner in which the process of the inventioncan be carried out using such other membrane configurations, based uponthe following explanation in terms of the hollow-fiber membranes.

The hollow fibers, typically 400 to 1200 of them, may be bundledtogether and secured in a cartridge, which is usually made of plastic.In the ultrafiltration process the process fluid, in this case thewhitewater, is passed through the hollow lumen of the fibers, and theultrafiltered permeate, as it passes through the active membrane layerand exits via the porous support structure of the fiber, is collectedwithin the body of the cartridge, from which it is then drained as anessentially polymer-free liquid.

In general, the polysulfone membranes may be operated under conditionswhich include a pH range of 1-14, a temperature from 0° C. to about 70°C., and a maximum pressure across the membrane of about 275 kPa. Themaximum temperature the membranes can withstand is about 70° C. and themaximum operating pressure is about 275 kPa. The membranes will nottolerate temperatures below 0° C. because the moisture entrained in thepolysulfone matrix will freeze and may rupture the membrane. Thepolysulfone membrane is not normally used with organic solvents, as themembrane surface may be damaged or the support structure actuallydissolved by certain solvents. Two different types of cartridges wereused in the examples below; the PM500-75 and the PM50-60. The PM500-75is more preferred. These cartridges are available from Romicon, Inc.,Woburn, Mass. 01801. The cartridges have the following characteristics:

PM500-75: 500,000 dalton MW cut-off 1.90 mm (75 mil) ID 4.83 m² ofeffective membrane area 0.01-0.03 μm pore size

PM50-60: 50,000 dalton MW cut-off 1.52 mm (60 mil) ID 6.13 m² ofeffective membrane area 0.003-0.008 μm pore size

Both cartridges have a 12.7-cm diameter and a length of 1.092 meters.The PM500-75 cartridge contains about 790 individual polysulfone fibersand the PM50-60 cartridge contains about 1250. The PM500-75 cartridgeshowed slightly better flow characteristics than the PM50-60 cartridge.

ULTRAFILTRATION SYSTEMS Components of a System

An ultrafiltration system consists of four major components; a source ofliquid to be treated, an ultrafiltration membrane, and a means formoving the liquid from the source to the membrane and generating apressure differential across the membrane. This is usually a pump orother pumping means. Ancillary components which may be added to thissimplest system include a pre-filter which removes from the liquid anyparticulate matter which is large enough to plug the constrictedportions of the system, e.g. the lumens of hollow-fiber membranes wherethese are employed. The simplest practical arrangement of thesecomponents is shown in FIG. 1. This set-up is termed straight batch.

A more elaborate embodiment of the apparatus of the invention is shownin FIG. 2, which illustrates the use of additional system components:the pre-filter (9) described above, a recirculation line (5) torecirculate whitewater from the whitewater outlet of the cartridge (7)back to the inlet of the pump (3), an auxiliary tank (10) that may beused instead of the main feed tank as the whitewater source when thevolume of the whitewater latex has been reduced significantly, andvarious control valves (12-24) which may be used to control theoperation of the apparatus during the process of the present invention.

The pump must be able to circulate the process fluid at a flow highenough to maintain the desired pressure at the inlet of the cartridge,200-275 kPa in the embodiment shown. A suitable pump is a horizontallymounted, centrifugal pump with a double mechanical water-flush seal, ora double-diaphragm pump such as the Wilden M-15 pump obtainable fromWilden Pump and Engineering Company, Colton, Calif., U.S.A. Thisparticular pump is a low-shear pump which is particularly advantageousin the process of the present invention; other pumps may be used if careis taken to maintain the low shear required for the process. In thepreferred embodiment, this pump should be able to pump cleaning solutionat about 415 liters per minutes per cartridge at an inlet pressure ofabout 200 kPa to insure adequate cleaning of the membrane.

Some whitewater emulsions are more sensitive to destabilization athigher temperatures. The pumping of the emulsion through theultrafiltration system adds energy to the emulsion, raising itstemperature, so a cooling means in the recirculation loop may bedesirable. One suitable cooling means would be a heat exchanger, andpreferably a heat exchanger that exchanges heat between the emulsion anda flowing, cooled liquid.

In whitewater ultrafiltration systems that concentrate the polymer latexto greater than 20 wt. % solids, the pressure applied to theconcentrated latex is preferably controlled to minimize shear of thelatex and help prevent its destabilization. The pressure and shear tendto increase as the solids content, and consequently the latex viscosity,increase. One way to control the applied pressure is to control thespeed of the pump; an alternative way is to bleed off excess pressure bybleeding whitewater from the pressurized portion of the system back intothe whitewater source.

The filter (9), a bag filter or similar filter placed in the whitewaterflow between the whitewater source and the whitewater inlet of thecartridge, is a preferred component of an ultrafiltration system whichhelps protect the hollow-fiber membrane from large particles that canplug the fibers or tear the membrane. The filter is typically positioneddownstream from the pump in the recirculation loop. The filter may alsobe placed upstream from the pump in the feed line, although anadditional pump may be needed to pump the whitewater from the feed tankthrough the filter. This option allows the filter to operate at lowerflow rates than when it is placed in the recirculation loop. The filtershould be such that it will retain particles large enough to plug thelumens of hollow-fiber membranes, or other constricted points in thesystem; in a preferred embodiment the filter is a screen with a meshsize that passes particles no larger than approximately half the size ofthe lumen diameters. Smaller-mesh filters are less preferred becausethey increase pressure drop and shear.

Minimizing the internal volume of the combined piping, filters, pump andcartridges, also termed the "holdup volume", and minimizing the fluidlevel in the feed tank, are important to the process and apparatus ofthe present invention. The holdup volume is preferably about 15% or lessof the initial volume of whitewater to be concentrated, more preferablyabout 10% or less, and still more preferably about 5% or less.

The whitewater source may be a tank or similar vessel, or other sourceof whitewater produced during cleaning of equipment for producing aparticular type of polymer latex, and stored for polymer recovery. Wherethe source is a tank or similar vessel, the bottom of the vesselpreferably has a conical shape to facilitate maintaining a reasonabledepth of latex, so that air is not drawn into the system by the pump.Alternatively an optional, smaller vessel (10) may be employed when thevolume of latex has dropped below a level practical for the largersource vessel.

Maintenance Cleaning

Periodically cleaning the membrane surface may increase the flux rate ofpermeate. The three processes described below are suitable for thiscleaning. Cleaning frequency is determined by the frequency at which theflux rate drops below a desired level, and a typical frequency ishourly. The desired flux rate is selected such that fouling of themembrane is still reversible; if ultrafiltration is continuedsignificantly past the desired minimum flux rate, the fouling becomesmore and more difficult to adequately remove, and if continuedsufficiently past the desired minimum flux rate, the fouling becomesessentially permanent; that is, the membrane cannot be restored to areasonable approximation of its initial flux rate even with extremecleaning measures. Increased difficulty in removing fouling increasesthe time required for cleaning, and thus the time the membrane is out ofservice. A preferred value for the desired minimum flux rate is about10% to 15% of the initial whitewater flux rate.

Reverse Flow/Recycle

Reverse flow/recycle is a combination of permeate recycling andreversing the direction of process-fluid flow. During recycle thepermeate flow from the cartridge is blocked, and the pressure outsidethe membrane fibers is allowed to rise until it is approximately equalto the average pressure inside the fiber lumens from inlet to outlet. Insuch a condition, the pressure is reversed across the membrane over atleast a portion of the length of the membrane; that is, a reversepressure differential is established across at least a portion of themembrane. When this condition is established, the permeate flows fromthe porous support side of the membrane fibers to the tube side in theexit lower pressure) portion of the fibers, where the reverse pressuredifferential is established, and from the tube side to the poroussupport side in the inlet (high pressure) portion of the fibers, wherethe pressure differential is in the normal direction, that is, the samedirection as when the whitewater is being treated.

The reversed flow of the permeate, i.e., from the porous support side tothe tube side of the membrane, cleans the inner surface of the fibers.This flow reversal occurs for a pre-defined time, during which automatedvalves also reverse the flow of the process fluid (whitewater) throughthe fibers. After the reverse flow/recycle sequence, the flow directionis opposite of the direction that it started and both ends of thecartridge have been cleaned by permeate recycle.

Vacuum-Flush Cleaning

Vacuum-flush cleaning is similar to reverse flow/recycle in that itinvolves the flow of permeate from the porous support side to the tubeside of the hollow fibers. This feature is possible with high pressurebleed systems. With the pump operating, automated valves alter the flowpatterns to create a negative pressure in the recirculation loop, whichforces the permeate, at atmospheric pressure, back through the poroussupport side to the tube side, cleaning the membrane surface. Note thatrecirculation flow across the membrane surface ceases during thiscleaning process. This process is also called "suction backwashing", andis described in, for example, U.S. Pat. No. 4,986,918.

Backflushing

Backflushing is similar to vacuum-flush cleaning except that thepermeate is forced, usually by a small pump, back through the poroussupport side into the tube side of the hollow fiber. In this case, theprocess fluid may continue to recirculate through the cartridges.

Chemical Cleaning

The ultrafiltration unit should be cleaned frequently to maintainreasonable flux rates. In batch operations, cleaning after every batchis typical, while with continuous systems a reasonable compromisebetween flux rate and unit down-time must be selected. Cleaning afterbatches also helps prevent cross-contamination of different products.

A cleaning solution useful for cleaning polymeric membranes of thepresent invention is 1% by weight of sodium hydroxide and 200 ppm byweight of sodium hypochlorite in deionized water. Surfactants may beused separately, or with a cleaning solution such as that describedabove. A preferred cleaning solution is sold under the trademark,Micro®, made by International Products Corp., Burlington, N.J. and isstated by the manufacturer to contain the following major components:Glycine, N,N'-1,2-ethanediylbis-(N-(carboxymethyl)-, tetrasodium salt;Benzenesulfonic acid, dimethyl-, ammonium salt; Benzenesulfonic acid,docecyl-, cpd with 2,2',2"-nitrilotris-(ethanol); andPoly(oxy-1,2-ethanediyl), α-(nonylphenyl)-ω-hydroxy. Typical cleaningtemperatures are elevated to the range of 40°-60° C. to increase thecleaning rate of the solution, and typical cleaning times are about 30to 60 minutes. The deionized water is used to prevent metal ions fromfouling the membrane over long period of time.

The following examples are intended only to further illustrate theinvention and are not to limit it except as it is limited in the claims.All percentages and ratios are by weight unless otherwise specified, andall reagents are of good commercial quality unless otherwise specified.

EXAMPLES 1-19

These examples are intended to illustrate the process of the presentinvention and its application to whitewaters from production of a widevariety of polymer latices. The polymer latices from which thewhitewater samples were generated are shown below in Table 1. In thattable the following abbreviations are used to indicate monomercomponents:

MMA--Methyl Methacrylate

MAA--Methacrylic Acid

EA--Ethyl Acrylate

BA--Butyl Acrylate

Bd--Butadiene

BMA--Butyl Methacrylate

Sty--Styrene

The product designations in Table 1 indicate products of Rohm and HaasCompany, Philadelphia, Pa. 19105. Those marked with ¹ are Rhoplex®products and those marked with ² are Polyco® products. Rohplex andPolyco are registered trademarks of Rohm and Haas Company.

                  TABLE 1                                                         ______________________________________                                        Polymer Latex Sources of Whitewater for Polymer Recovery                      Exam-           Major Polymer                                                 ple   Product   Components   Application                                      ______________________________________                                         1    E-1381.sup.1                                                                            MMA/BA       Binder for architectural                                                      coatings                                          2    AC-235.sup.1                                                                            MMA/BA       Binder for architectural                                                      coatings                                          3    AC-261.sup.1                                                                            MMA/BA       Vehicle for architectural                                                     coatings                                          4    E-1698.sup.1                                                                            MMA/EA       Vehicle for architectural                                                     coatings                                          5    AC-417.sup.1                                                                            MMA/EA       Vehicle for architectural                                                     coatings                                          6    E-2091.sup.1                                                                            MMA/BA       Binder for architectural                                                      coatings                                          7    E-2003.sup.1                                                                            MMA/BA       Binder for architectural                                                      coatings                                          8    E-2437.sup.1                                                                            BA           Vehicle for adhesives                             9    N-1031.sup.1                                                                            BA           Vehicle for adhesives                            10    B15J.sup.1                                                                              MMA/EA       Binder for non-woven                                                          textiles, paper coating.                         11    TR-407.sup.1                                                                            MMA/EA       Binder for non-woven                                                          textiles                                         12    NW-1402.sup.1                                                                           EA           Binder for non-woven                                                          textiles                                         13    HA-16.sup.1                                                                             MMA/EA       Binder for non-woven                                                          textiles                                         14    2150.sup.2                                                                              Vinyl Acetate                                                 15    WL-91.sup.1                                                                             Styrenated   Binder for industrial                                            Acrylic      coatings                                         16    E-1421.sup.1                                                                            MMA/BA/Sty   Vehicle for floor polish                         17    ASE-75.sup.1                                                                            MMA/EA/MAA   Thickener                                        18    TR-934.sup.1                                                                            EA/BA        Binder for non-woven                                                          textiles                                         19    EC-1791.sup.1                                                                           MMA/BA       Binder for roof mastics                          ______________________________________                                    

These polymer latices represent a wide variety of latex products,incorporating a variety of surfactants, including anionic, nonionic andcationic surfactants. As will be shown below, polymer can be recoveredfrom the whitewater generated during cleanup of equipment used formanufacturing each of these latices, and the recovered polymer hasproperties that are close enough to the original polymer latex productthat the recovered polymer may be blended into the polymer latex productwithout degrading the performance characteristics of the product in theintended applications.

Whitewater Source

A drain tank rinse (whitewater) was pumped from a plant drain tank by aWilden diaphragm pump (obtained from Wilden Pump and EngineeringCompany, Colton, Calif. USA), through an 800-μm strainer into a1325-liter tote which was then stored until ultrafiltration.

Prefiltering the Whitewater

Whitewater from drain-tank flushes was prefiltered through an 800-μm bagfilter or a Johnson sock filter as the whitewater was pumped from thetote to the feed tank by a diaphragm pump, to filter out any polymerskin or gel which may have formed on the liquid surface when the toteswere stored.

Pilot Unit

The pilot unit consisted of a 4550-liter feed tank, a 380-literauxiliary feed tank and a two-cartridge, skid-mounted ultrafiltrationunit. The main components of the ultrafiltration unit were thefollowing:

Horizontally mounted centrifugal pump with a double mechanicalwater-flush seal, rated for 7460 Joule/second (10 horsepower) at 1750RPM, capable of pumping 530 liters/minute against a 270 kPa head.

Speed control for the pump.

Bag filter positioned after the pump in the recirculation loop.

Two 1.09-meter-long, 12.7-cm-diameter membrane cartridges.

Recirculation loop with option of:

a. High pressure bleed

b. Low pressure bleed

Automated reverse flow/recycle cleaning.

Automated vacuum-flush cleaning.

A schematic of the ultrafiltration unit is shown in FIG. 2.

The auxiliary feed tank was used at the end of a run to avoid turbulenceand splashing which occurred in the 4550-liter feed tank when the volumewas smaller than 380 liters.

Bag Filter

The bag filter was located in the recirculation loop between the pumpand the membrane cartridges. With dilute whitewater, a filtration rateof 760 liters/minute could be maintained. When 200 μm and 400 μm filterbags were used, they fouled badly, dropping the cartridge inlet pressurebelow 140 kPa; using 800 μm bags avoided this fouling.

Both PM50-60 and PM500-75 membranes were tested. The PM500-75 membranewas preferred for ultrafiltering whitewater because it allows highercross-flow at higher whitewater viscosity than the smaller PM50-60membrane.

Start-up

For start-up, the unit was flooded with water, trapped air was bled off,and the pump speed was increased until the desired cartridge inletpressure was reached.

Trapped air was bled by opening vents on the upper cartridge manifoldand the bag filter while the pump was running. To protect the membranes,the permeate ports on the porous support side of the cartridges werekept closed during this bleed. Once the desired cartridge inlet andoutlet pressures were reached, the permeate ports were re-opened.

Under normal operation, the permeate flows into the vacuum flush tank,through the cleaning tank and to a drain.

Operating Parameters

The inlet and outlet cartridge pressure was maintained at 210 kPa and 35kPa, respectively. At high solids levels, greater than 30 weightpercent, the viscosity increased rapidly, which increased the inletpressure on the cartridges and the pressure drop through the cartridge.The pump speed was controlled to maintain the inlet pressure at 200-240kPa. The outlet pressure was controlled with valves in the recirculationloop and the low pressure bleed line.

Most products which were ultrafiltered to high solids concentrationrequired pump speed-control settings of about 85-100% of maximum,although a few of the highly viscous products required 80% settings tomaintain the 210 kPa inlet pressure. Most ultrafiltration runs began attemperatures in the range of 15°-25° C. and ended at 30°-45° C., andrequired 2 to 5 hours for completion.

After the cartridge operating pressures were set, the system was eitherleft at a manual setting, without any maintenance cleaning, or set toautomatic, with the reverse flow/recycle and/or the vacuum-flushcleaning feature activated.

Shut-down

When the desired solids concentration was reached or the retentatevolume dropped to 130 liters, the system pump was turned off and theunit was drained and sampled for blending and lab analysis. Theautomated reverse-flow valves were activated and the system was shut offwith the valves in their half-way positions to allow complete drainage.

Water Flush

After the system had completely drained of concentrated whitewater, itwas set to its cleaning mode and flushed with water, or with thepermeate that had collected in the cleaning tank during processing. Inthe cleaning mode, the system feeds from the built-in cleaning tank,thus segregating the whitewater feed tank from the unit. The unit wasflushed for a total of 5 to 10 minutes, then drained as with theconcentrated whitewater.

Cleaning

The unit was cleaned with an aqueous, 1% sodium hydroxide solution,optionally with a small amount of Micro® cleaning solution (describedabove), prepared using deionized water, at 30°-60° C. During cleaningthe inlet and outlet cartridge pressures were the same as with normalprocessing, 210 and 35 kPa, respectively. The permeate valves wereclosed to place the cartridges in recycle, and the unit could be set tomanual or automatic mode. In automatic mode the flow direction reversedevery 15 minutes.

To clean residual caustic from the membranes and piping after thecleaning solution was drained, the unit was flushed with tap water, andtap water was circulated through the system at the same operatingpressure as in normal processing, but with the cartridges set torecycle.

Using the above-described procedure and equipment, 19 separate batchesof whitewater from the production of the polymer latices of Examples1-19 were treated to recover the polymer.

Cartridge Flow Data

At high solids, the flow through the cartridges was low enough that theinstantaneous flux could be roughly measured by placing the unit instraight batch and measuring the return flow rate. The results are for aPM500-75 mil membrane at 275 kPa inlet and 55 kPa outlet cartridgepressure are shown below:

                  TABLE 2                                                         ______________________________________                                        Flux and Cartridge Flow Rate for Typical Whitewater Latices                   Brookfield   Cartridge   Solids  Approx.                                      Viscosity    Flow Rate   (wt.    Flux                                         (Centipoises)                                                                              (Liters/minute)                                                                           %)      (liters/m2/day)                              ______________________________________                                        Exam-  92         68         45.7  120                                        ple 18                                                                        Exam- 100        102         52.7  120-160                                    ple 19                                                                        Exam- 103        151         46.2  120-160                                    ple 14                                                                        ______________________________________                                    

As may be seen from Table 2, even at relatively high solids levels,where concentration polarization would be expected to cause low fluxrates, the observed rates are high enough to be practical.

Similarly, Table 3, below, shows that the wide range of whitewateremulsions described in Table 1 may be ultrafiltered according to theprocess of the present invention, at practical flow rates. In Table 3,the average flux value is determined by averaging the flux through oneof five PM500-75 cartridges (described above) over the solids range from4% to 30%,

                  TABLE 3                                                         ______________________________________                                        Individual Average Fluxes for 4 to 30 wt. % Solids                                   Car-    Avg. Flux  Exam- Car-  Avg. Flux                               Example                                                                              tridge  liters/m.sup.2 /day                                                                      ple   tridge                                                                              liters/m.sup.2 /day                     ______________________________________                                        Architec-                 Textile                                             tural                     Bind-                                               Coatings                  ers                                                 1      1       1426             5     1467                                           2       1263       11    1     1548                                    2      3        937             2     1548                                           4        937       12    1     1222                                    3      4        937             2     1222                                           5       1670       13    3      652                                    4      3       1507             4      896                                           4        855       14    1     1345                                           4        774             2     1263                                    5      3       1181       Avg. Flux = 1182 ±                               6      4       1140       650 liters/m.sup.2 /day                                    4        733       Other                                                      5       1222       19    1     1670                                           1       1018             2     1508                                           2        896       15    4      570                                    7      4        978       16    4      693                                           5       1548             5      774                                    Avg. Flux = 1100 ±                                                                           17      1       1263                                        570 liters/m.sup.2 /day   2       1263                                        Adhe-                                                                         sives                                                                         8      4       2485                                                           9      1       2404                                                                  2       2404                                                           Avg. Flux = 2445 ±                                                         80 liters/m.sup.2 /day                                                        ______________________________________                                         Note: Error is ± Two Standard Deviations                              

Applications Testing

Applications testing was conducted by blending polymer recovered by theprocess of the present invention into samples of the polymer latex fromwhich the whitewater was generated, and subjecting the blends to thesame tests used to determine the suitability of the polymer latex forits intended application. Thus the materials recovered byultrafiltration according to the present invention of whitewateremulsions from Examples 3 and 4 were tested by blending them, at levelsof 1% and 5% by weight, into the original products of Examples 3 and 4and testing the paint performance of the resulting polymer latex; noadverse effects on paint performance properties were seen.

Similarly, the recovered material of Example 10 was tested at similarblend levels for gloss, brightness, opacity and surface pick strength instandard paper-performance testing; no adverse effect was seen.

Similarly, the recovered material of Example 11 was tested at similarblend levels for wash durability by spraying the polymer latex onto apolyester web and laundering it under household laundry conditions forfive cycles; no adverse effect on durability was seen.

Similarly, the recovered material of Example 12 was tested at similarblend levels for tensile strength and stretch on a simulated "wet-wipe"non-woven fabric. Another test of tensile strength that involved soakingthe fabric in a simulant for the consumer lotion used with wet wipes andcomparing the results after a 1-hour soak and after soaking at 49° C.for 10 days was run using the material of Example 12 at the 1% and 5%blend levels; no adverse effect on any of these properties was seen.

Similarly, the recovered material of Example 16 was tested at similarblend levels for floor-polish properties; no adverse effect was seen.

Similarly, the recovered materials of Examples 1,3, 4, 6 and 7 weretested at similar blend levels as emulsions for the followingproperties: freeze-thaw stability by exposing to 5 cycles of freezingand thawing; heat aging, by heating to 60° C. and holding at thattemperature for 10 days; and mechanical stability, by agitating in aWaring Blendor® mixer for five minutes at high speed. These samematerials were blended into standard paint formulations and tested forgloss, adhesion, color acceptance and the freeze-thaw stability and heatageing described above. Except for the color acceptance tests for thematerials of Examples 1 and 6, for which no data are available, noadverse effect was seen on any of these properties for any of thesematerials as recovered emulsions or as paints formulated from therecovered emulsions.

Emulsion Stability

Samples of the final, concentrated, ultrafiltered emulsions weretransferred to glass jars and allowed to settle by gravity for twoweeks. To compare the stability of the ultrafiltered product of thepresent process with the whitewater, samples of whitewater were preparedby diluting the original polymer latex product to 5% solids withdeionized water. These were also allowed to settle by gravity for twoweeks. The results of this test, reported as centimeters of clear liquidlayer formed above the emulsion after the time periods indicated, areshown below in Table 4. The ultrafiltered product of the present processwas observed to be at least as stable as, and frequently more stablethan, the whitewater from which it was prepared.

                  TABLE 4                                                         ______________________________________                                        Settling of Ultrafiltered Emulsion                                                   Ultrafiltered Emulsion                                                                     5% Whitewater Emulsion                                    Example  1 Week   2 Weeks   1 Week  2 Weeks                                   ______________________________________                                        1        0        0.2       1.3     2.5                                       2        0        0                                                           3        0        0         0       0                                         4        0        0         1.0     1.3                                       5        0        0.2                                                         6        0        0                                                           7        1.3.sup.1                                                                              1.3       1.3.sup.1                                                                             1.9                                       8        0        0.2                                                         9        0        0                                                           10       0        0         0       0                                         11       0        0                                                           12       0        0         0       0.3                                       13       0        0                                                           14       0        0         0       0                                         ______________________________________                                         .sup.1 After one day                                                     

The above results show that the polymer latex recovered by the processof the present invention is capable of being blended into undilutedpolymer latex at significant levels without deleteriously affecting theimportant properties of the undiluted latex. Such a capability meansthat the polymer in the whitewater emulsion, which heretofore hasrepresented only waste to be disposed of, or a low-grade byproduct, maynow be returned to the product stream, significantly increasing theyield of the process and significantly reducing the environmentalproblem of waste disposal.

Although the invention has been described with regard to its preferredembodiments, which constitute the best mode presently known to theinventors, various changes and modifications which would be obvious toone having ordinary skill in this art may be made without departing fromthe scope of the invention which is defined by the claims.

We claim:
 1. An apparatus for concentrating whitewater emulsions whichcomprises(a) one or more ultrafiltration cartridges (1) having disposedtherein one or more ultrafiltration membranes having an active membraneside, the cartridges further having a whitewater inlet (6) and awhitewater outlet (7) such that a liquid flowing between the inlet andoutlet will flow, in the laminar domain, in contact with the activemembrane side of the membrane, and the cartridges further having apermeate outlet, the membrane separating the permeate outlet from liquidflowing between the whitewater outlet and the whitewater inlet, (b) awhitewater inlet line for the flow of liquid, the line having adischarge end and an input end, the discharge end being connected to thewhitewater inlet of the ultrafiltration cartridges, (c) a pumping means(3) for a liquid, the pumping means having an inlet and an outlet, theoutlet of the pumping means being connected to the input end of thewhitewater inlet line and the inlet of the pumping means being connectedto a source of whitewater (4), thereby providing a flow of whitewaterfrom the source of whitewater sequentially through the pumping means(3), the whitewater inlet line, the whitewater inlet (6), theultrafiltration cartridge and the whitewater outlet (7), (d) a returnline (5) for the flow of whitewater connected from the whitewater outlet(7), of the ultrafiltration cartridges to the source of whitewater (4),the inlet of the pumping means (3), or the connection between them,thereby providing a path for liquid flow sequentially from the inlet ofthe pumping means through the pumping means (3), the whitewater inletline, the ultrafiltration cartridges (1) from the whitewater inlet (6)to the whitewater outlet (7) of the cartridges and thence through thereturn line (5) back to the source of whitewater, the inlet of thepumping means, or the connection between them, forming a recirculationloop, and (e) a first pressure control means disposed within therecirculation loop between the inlet of the pumping means and thewhitewater inlet of the cartridges for controlling the pressuredifferential between the whitewater inlet and the whitewater outlet ofthe cartridges to maintain flow across the ultrafiltration membranes inthe laminar domain and to maintain a total shear on the whitewateremulsion in the entire apparatus below a level which will destabilizethe whitewater emulsion.
 2. The apparatus of claim 1 wherein the firstpressure control means is a speed control for the pumping means.
 3. Theapparatus of claim 1 wherein a filter is disposed in the whitewaterinlet line to filter liquid flowing from the pumping means to thewhitewater inlet of the cartridge.
 4. The apparatus of claim 1 wherein asecond pressure control means is disposed within the recirculation linefor controlling the pressure differential between the whitewater outletof the cartridge and the atmosphere.
 5. The apparatus of claim 1 whereina cooling means is disposed within the recirculation loop.
 6. Theapparatus of claim 1 wherein the ultrafiltration membranes areconfigured as hollow fibers.
 7. The apparatus of claim 6 wherein thehollow fibers are polysulfone hollow fibers.